The present invention relates to a sheath heater, especially for use in a sheath-type glow plug for diesel engines.
The technology of modern diesel engines places great demands on sheathed-type glow plugs, especially with regard to size, sturdiness, rapidity of heating-up, and resistance to high temperatures. It is usually desirable that, at a heater output of roughly 70 to 100 W, a temperature of 1000xc2x0 C. and a steady-state temperature of 1200xc2x0 C. can be achieved within 2 seconds.
Conventional sheathed-type glow plugs have metallic and ceramic heaters. Customary designs of ceramic sheathed-type glow plugs have internal metallic or ceramic heaters, which are sintered into a nonconductive ceramic that is stable at high temperatures. However, sheathed-type glow plugs having this type of design can only be manufactured using expensive heat pressing methods. On the other hand, sheathed-type glow plugs having external heaters made of composite ceramics can be manufactured using simpler and more cost-effective sintering methods.
A diesel-engine glow plug having a cylindrical metal tube, a connecting device for the electoral contact, and a ceramic heating device, is described in, for example, PCT Application WO 96/27104. In this glow plug, the cylindrical metal tube at its tip supports the ceramic heating device in a floating manner, the ceramic heating device being contacted using the connecting device, so that during the glow process a current flows through the ceramic heating device.
In this context, the ceramic heating device has at least one location having a reduced cross-section, the reduction of the cross-section of the ceramic heating device occurring at the location at which the fuel-air mixture strikes. The cross-section reduction in this ceramic heating device is realized such that the thickness of the lateral wall is correspondingly reduced at the location in question.
In a sheathed-type glow plug of this type, it is possible that the area of the heating device that is most accessible to the combustible mixture reaches the necessary ignition temperature the most rapidly due to the resulting greater resistance. As a result, shorter heating-up times are possible for the sheathed-type glow plugs. A defined reduction of the wall thickness of this magnitude makes it possible to bring to the highest temperature precisely the location of the sheathed-type glow plug where the combustion mixture strikes.
In PCT Application No. WO 00/35830, a further conventional solution is described for creating a rapidly self-heating sheath heater, achieving this by reducing the cross-section of the sheath heater in the area of the hot zone. A sheath heater of this type, for the purpose of cross-section reduction, is configured having a filigree tip.
Conventional sheath heaters of this type have the disadvantage that they have a hot zone that must be created in an extremely finely fashion by forming a pointed tip or otherwise reducing the cross-section in the area of the tip of the sheath heater, in order to be able to be heated rapidly to a high temperature.
However, filigree tips of sheath heaters, that are therefore only capable of standing up to small stresses, are extremely sensitive and can be easily damaged, especially during handling, installation in the engine, etc.
Furthermore, areas of sheath heaters that are reduced in their cross-section in this manner also have an insufficient thermal mass, so that it is impossible to achieve satisfactory temperature stability, and therefore in response to a sudden cooling in the environment, such as during a cold start of the engine, the danger of blowing out the sheathed-type glow plug is very great.
In accordance with an example embodiment of the present invention, a sheath heater in a sheathed-type glow plug for diesel engines may have the advantage that, as a result of the changed shape of the tip of the sheath heater, it is possible to achieve significantly greater mechanical stability, because the tip of the sheath heater is not reduced in its overall cross-section.
In addition, the heater tip may have a greater thermal mass. This has the effect, under certain operating conditions, specifically in a cold start, of working against a blow-out of the sheathed-type glow plug.
According to one example embodiment of the sheath heater, the latter is configured so as to be generally rotationally symmetrical. This may be advantageous because, as a result of a sheath-heater configuration of this type, it is possible that the glow plug glows in its central tip area, as is required for modern, direct-injection diesel engines.
In this context, in configuring the sheath heater, it can be provided that the insulation layer is generally surrounded by the conductive layer.
It has been demonstrated that it is advantageous, especially for the production of the sheath heater, if the insulation layer is surrounded by the conductive layer in a generally sandwich-like manner, i.e., if the cross-section includes a sequence of conductive layer, a central insulation layer, and once again a conductive layer, the insulation layer being situated at least approximately in a central area of the cross-section of the sheath heater.
It may be advantageous if the sheath heater is manufactured by injection-molding, and if the insulation layer is injection-molded first, the insulation layer extending, in its edge area, i.e., the area not bordering on the conductive layer, at least in part right to the periphery of the sheath heater. As a result, the insulation layer can be placed in a tool so the conductive layer can be sprayed on, for example, perpendicular to the tool parting plane.
In particular, with regard to the size of the sheath heater, which may be kept very small, it may be advantageous if the sheath heater has a diameter in the range of roughly 2 mm to 5 mm.
It is expedient if the arrangement of the conductive layer and the insulation layer is optimized for the specific manufacturing process of the sheathed-type glow plugs. Preferred manufacturing processes are injection molding and/or injection pressing. The optimization advantageously takes place using analytic processes, in particular, using a finite-element process. Using an optimization of this type, it is possible to calculate a geometry of the sheath heater which can be produced very simply and cost-effectively using a two-stage injection-molding process, without reworking and subsequent sintering.
In this context, it is preferred if the ceramic composite structure of the conductive and insulation layers has as constituents tri-silicon tetra nitride and a metallic silicide. In this context, it is greatly preferred if the ceramic composite structure for the conductive layer be made of 60 wt. % MoSi2 and 40 wt. % Si3N4, as well as sintering additives, and for the insulation layer to be made of 40 wt. % MoSi2 and 60 wt. % Si3N4, as well as sintering additives.